1. Field of the Invention
The present invention relates to an image-forming device such as a laser printer.
2. Description of Related Art
There have been proposed electro-photographic image forming devices of a type that uses a non-magnetic, single-component developer agent. Generally, a developing cartridge is detachably mounted in the image forming device of this type. The developing cartridge includes: a toner accommodating chamber, a supply roller, a developing roller, and a layer thickness regulation blade.
In the developing cartridge, an agitator is disposed in the toner accommodating chamber. The supply roller is provided facing the developing roller. The supply roller is in contact with the developing roller. The layer thickness regulation blade is pressed against the surface of the developing roller.
The toner in the toner accommodating chamber is supplied to the supply roller by the agitator. As the supply roller rotates, the toner is supplied from the supply roller to the surface of the developing roller. When the toner is between the supply roller and the developing roller, the toner is electrically charged due to friction between the supply roller and the developing roller. As the developing roller rotates, the toner on the surface of the developing roller enters the gap between the layer thickness regulation blade and the developing roller. While the toner is in the gap between the layer thickness regulation blade and the developing roller, the toner is further electrically charged due to friction between the layer thickness regulation blade and the developing roller. The toner is formed or shaped in a layer of a certain amount of thickness and is supported on the surface of the developing roller.
The developing cartridge is mounted in the image forming device, with the developing roller facing a photosensitive drum in the image forming device. An electrostatic latent image is formed on the photosensitive drum. The developing roller is applied with a certain amount of developing bias voltage. Accordingly, when the toner layer on the developing roller faces the photosensitive drum, the toner layer develops the electrostatic latent image into a visible toner image. The toner image is transferred onto the paper. Thus, a visible image corresponding to the electrostatic latent image is finally formed on the paper.
Generally, the image-forming device that uses the non-magnetic, single component developer agent exhibits the relationship between the effective developing bias voltage and the transmission density as shown in FIG. 1.
The “effective developing bias voltage” is a potential difference between the developing roller and the electrostatic latent image formed on the photosensitive drum. In other words, the “effective developing bias voltage” is a difference between the developing bias voltage applied to the developing roller and the electric potential at the portions of the photosensitive drum where the electrostatic latent image is formed.
The “transmission density” is a quantity that is proportional to the amount of toner adhering to the paper per unit area.
As shown in FIG. 1, when the effective developing bias voltage is relatively low, the transmission density changes in proportion to the effective developing bias voltage. However, when the effective developing bias voltage becomes relatively high, the transmission density is saturated and is maintained at a constant value regardless of whether the effective developing bias voltage changes.
Japanese patent Laid-Open No. 2003-43761 has proposed an image-forming device, which exhibits the developing characteristics of FIG. 1 and which stabilizes its development by setting the effective developing bias voltage to have a sufficiently high value that allows the transmission density to be saturated at the constant value.